Calcobutrol is an additive in the galenic formulations of Gadobutrol and solves the problem of preventing the release of free gadolinium in the formulations (solutions). Gadobutrol is a gadolinium-containing contrast agent for nuclear spin tomography and has been allowed in Germany since 2000 as Gadovist® for the indication “contrast enhancement by cranial and spinal magnetic resonance tomography (MRT)” (EP 0 448 181 B1, EP 0 643 705 B1, EP 0 986 548 B1, EP 0 596 586 B1, and CA patent 1341176). Gadobutrol is a non-ionic complex, consisting of gadolinium(III) and the macrocyclic ligand 10-(2,3-Dihydroxy-1-(hydroxymethyl)propyl)-1,4,7,10-tetraazacyclodecane-1,4,7-triacetic acid (butrol). Gadovist is sold as a 1 molar aqueous solution and has the following components in the formulation: Gadobutrol, Calcobutrol sodium salt, Trometamol, hydrochloric acid and water for injection.
It has been found that for most gadolinium-containing contrast agents it is an advantage to apply an excess of the complex-forming ligand in the formulation in the form of the calcium complex (EP 0 270 483 B2). The role of the calcium complex is to prevent the release of free gadolinium in the formulation (e.g. by storage for several years, re-complexation with foreign ions from the glass).
The synthesis of the calcium complex (Calcobutrol) is described in detail in Inorg. Chem. 1997, 36, 6086-6093. The process disclosed therein, however, does not provide Calcobutrol with the purity required by the authorities. An exact reproduction of the process of Scheme 3 (page 6088-6089) results in a material with a purity of only about 94%, as measured by HPLC (stationary phase: Hypersil phenyl (5 μm) from SHANDON; mobile phase: acetonitrile/borate buffer (pH 8) in the vol. ratio 20/100; detection: UV detector (200 nm); injection volume: 10 μl). The ligand available from the synthesis of Gadobutrol (butrol) does not have the required high purity for directly transferring it to the calcium complex. A further purification of the ligand is difficult due to the zwitterionic nature of the ligand. Unlike the ligands BOPTA, DTPA, and DOTA, which crystallise at a pH of 1.7-1.8 (according to U.S. Pat. No. 5,595,714), it is not possible to crystallise butrol at any pH (see comparative Example below) and thus not possible to purify it by crystallisation. Without being bound by a specific theory, the difference in ability to crystallise is ascribed to the dihydroxy-hydroxymethyl-propyl sidechain in butrol, which is not present in any of BOPTA, DTPA, or DOTA. It is likely that the lack of crystallisation is due to a difference in polarity or the ability to form hydrogen bonds. Finally, another possible reason could be the so-called “glycerol effect” from the dihydroxy-hydroxymethyl-propyl sidechain, i.e. the ability of glycerol to prevent crystallisation of water at 0° C., disrupting hydrogen bonds in the water crystals.
While the neutral gadolinium complex (Gadobutrol) can be purified in an ion exchange column (such as Amberlite IRC 50, Amberlite FPC3500, or Amberlite IRA 67) and subsequently obtained in very high purity (>99.7%) through a very efficient crystallisation (e.g. from ethanol, preferably with less than 200 ppm water), this is not possible for Calcobutrol because of the extra acid functionality. A purification of the calcium complex was unsuccessful since even by preparative HPLC there was an impurity very close to the main peak that could not be separated. Several different ways of separating Calcobutrol by HPLC were attempted (varying mobile phases, gradients etc.), but none of them accomplished the separation.
The thermodynamic stability constant of Calcobutrol and the acid dissociation constant have been determined by pH-potentiometric equilibrium titrations of the ligand (butrol) in the presence of Ca2+ ions (at 25° C., in 0.1 N KCl) at different ratios of Ca2+:ligand. The results are:log(KCaL-)=14.67±0.02 KCaL-=[CaL−]/[Ca2+][L3−]pKa=3.39±0.12 Ka=[CaL−][H+]/[CaLH]
Based on these measured values, the distribution of calcium between free calcium ion, neutral complex (Calcobutrol, ligand has two negative charges), and anionic complex (ligand has three negative charges) can be calculated for different pH values. The result is presented in FIG. 1. It is evident that the neutral complex does not constitute more than 20% of the calcium-containing species at any pH. Due to this equilibrium between the calcium-containing species, preparative methods in aqueous solution will lead to some impurities.
While the anionic complex is the dominant species at higher pH values, this is not very useful for purification purposes. The salts with this complex (e.g. the sodium salt) are not suitable for work-up. The sodium salt of the complex is a highly hygroscopic, glassy material, which cannot be handled at any useful scale. In the preparation of the Gadovist solution, the sodium salt is therefore prepared in situ by adding sodium hydroxide to Calcobutrol.
The large difference in stability between Gadobutrol and Calcobutrol is what makes Calcobutrol useful in the Gadovist formulation in the first place, i.e. the large difference in stability between the gadolinium complex and the calcium complex means that the calcium complex will scavenge any free gadolinium ions by forming the gadolinium complex.
It is an object of the present invention to obtain very pure Calcobutrol in the highest possible yield, preferably in crystalline form.